1. Field of the Invention
The present invention relates to a read-out apparatus of a solid state imaging device and an image scanner using the read-out apparatus.
2. Description of the Related Art
A CCD (Charge Coupled Device), which is an example of solid state imaging devices used for image scanners, is provided with a large number of sensor elements for converting light to electricity (named photoelectric conversion) and for storing photoelectric charges obtained by the photoelectric conversion and an analog shift register for transferring the charges stored in the sensor elements and for successively outputting a voltage signal. The voltage signal outputted from the analog shift register is sampled and then converted into a digital signal.
By using a CCD linear image sensor, in which sensor elements are disposed in line, when an original in the letter size (8.5 inch width) is picked-up with a resolution of 400 DPI (Dot Per Inch), the number of sensor elements, namely the number of photoelectric converter elements, is 8.5 inches.times.400 elements/inch=3400 elements. However, the number of elements of CCD linear image sensors which are currently available and which meet applicable standards is one of 1024, 2048, 2592, and 5000. Thus, to satisfy 3400 elements, it is necessary to use a CCD linear image sensor with 5000 elements.
A conventional read-out apparatus is designed to scan all the photoelectric converter elements of the CCD image sensor at a particular speed. Thus, if image information inputted to the CCD linear image sensor with 5000 elements is read out with the conventional read-out apparatus, the photoelectric converter elements which are not used for picking-up the image are also scanned and thereby the unnecessary read-out time is required. In other words, in case that image of an original which can be satisfactorily picked-up by a CCD sensor with 3400 elements is picked-up by a CCD sensor with 5000 elements, 1600 elements are excessively read out causing the read-out time to unnecessarily increase.